The estrogen receptor (ER) plays an important role in maintaining cellular homeostasis; however, the ER also plays a critical role in formation and growth of estrogen-dependent mammary and endometrial cancers which are major health problems for women. Preliminary studies in this laboratory have demonstrated that E2-induced transactivation of several genes in breast cancer cells involves interaction of an Sp1/ER complex with GC-rich motifs. Therefore, we hypothesize that interaction of the Sp1/ER transcription factor complex with 5'-(promoter) GC-rich sequences plays an important role in hormone-induced expression of diverse genes, including those involved in DNA synthesis. Other factors and enhancer sequences may modulate this response, and Aims 1-4 will therefore extensively investigate the molecular mechanisms of Sp1/ER- induced expression of several target genes containing GC-rich 5'- promoter regions. Aim 1 will investigate the requirements for formation and function of Sp1/ER-DNA complexes using Sp1(n)xERE(1/2) 5'-promoter sequences and oligonucleotides with variable DNA binding sites and intervening oligonucleotides; the interaction of Sp1/ER with other cellular co-activators including ERAP-140, ERAP-160/SRC-1, RIP 140 and CBP/p300 will also be investigated. Preliminary results have shown that ER and Sp1 proteins physically interact and E2-induced gene expression can be observed through promoters containing only GC-rich Sp1 binding sites. Therefore, Aim 2 will investigate the functional synergy and physical interactions of Sp1 and ER proteins in which transactivation is independent of ER-DNA interactions. The proposed studies will determine domain-specific interactions between Sp1 and ER proteins, ligand (antiestrogen vs estrogen)-dependent differences in mechanisms of transactivation and the role of co-activators in the induction process. Aim will focus on the role of Sp1 and GC-rich sequences in the mechanism of E2-induced IGF receptor, MUC-1 and ornithine decarboxylase gene expression. Several hormone-induced genes involved in DNA synthesis, including dihydrofolate reductase, thymidine kinase, thymilidate synthase and adenosine deaminase, contain GC-rich regions in their 5'-proximal promoter regions, and Aim 4 will determine the role of Sp1/ER complexes in mediating induction of these genes. The proposed research will investigate the molecular mechanism which regulate several E2-responsive genes that play an important role in proliferation of breast cancer cells. These results will ultimately facilitate design of chemotherapeutic approaches which target specific genes.